The background of the invention will be described on the use of the string for the racket string and the fishing line, which are the principal uses of the string of the present invention.
Sheep gut had been commonly used for the racket string until synthetic materials were developed and twisted strings mainly of nylon filaments replaced the sheep gut racket string. Typical synthetic racket strings consist of a single thick core and two layers of thirty or a single layer of sixteen to eleven thin filaments overlayed around the core filament. The combination of the core filament and the wrapping thin filaments are integrated by twisting, then subjected to resin treatment so that the core filament and the wrapping thin filaments are mutually bonded together.
The specific size of typical racket strings is shown below.
______________________________________ CORRESPON- DIA- DEN- DING STRING USE OF THE STRING METER IER GAUGE ______________________________________ Tennis racket string 1.47mm 17600 No. 80 type A: Tennis racket string 1.28mm 13200 No. 60 type B: Badminton racket string: 0.81mm 5820 No. 24 ______________________________________
The twist number is 2 t.p.i. for the respective strings.
Phenol resin is most commonly used for bonding the component filaments of the string. Phenol resin is applied to the string during the twisting process. The tensile strength of the string bonded by phenol resin is reduced to 70% of the tensile strength of a standard nylon monofilament as the surface of the component filaments of the string is eroded by the erosive action of the phenol resin. The surface of the external component filaments of thus bonded string also is roughened by the erosive action of the phenol resin so that the roughened surface may be damaged and fall off during the stringing of the racket. In order to prevent this damage, it is usual to apply protective coating of nylon resin to the surface of the phenol resin bonded string, however, these treatments reduce the specific strength of the component synthetic material and deteriorate the surface tenacity of the component filaments while reducing the sharpness of the spiral grooves, the frictional resistance of which is essential to the racket string. The intrinsic resistance of sheep gut effectively controls the ball and the resistance of the special grooves of the synthetic string produced by twisting the string corresponds to the intrinsic surface resistance of the sheep gut; therefore, degradation of the sharpness of the special grooves causes the performance of the synthetic string to reduce.
The surface twist grooves, high tensile strength and good flexibility are essential properties for the strings for fishing nets and fishing lines. Therefore, twisted strings are used for these purposes.
The string according to the present invention as hereinbefore described is a multi-filament string integrated by twisting into a substantially single string having a tensile strength exceeding the standard tensile strength of the equivalent nylon mono-filament by 1 to 3 Kgs, the intrinsic surface tenacity of the material, the original sharpness of the spiral twist grooves and high flexibility. These properties of the string of this invention provide a highly strong and durable racket string which effectively controls tennis balls. It will be well understood that the string according to the present invention is sufficiently provided with properties that satisfy the requirments of fishing nets and fishing lines.
Furthermore, although the string according to the present invention is constituted of twisted multi-filaments, the component filaments will not be damaged and broken separately, which is liable to occur in the conventional bonded multi-filament string because the component filaments are integrated in the form of a mono-filament. Thus, the string of the present invention has no possibility that the component filaments are damaged and broken separately.
Still further, when a hollow filament is used instead of a solid filament as the thick core filament, the flexibility of the string is improved still more and if the internal hollow space of the hollow core filament is filled with a dye coloured oil, liquid, fluorescent paint or oil or liquid containing suspended golden or silvery aluminium powder, the string will show beautiful appearance, which is expected to provide a great fish luring effect when the string is used for fishing nets.
Accordingly, an object of the present invention is to provide a twisted synthetic multi-filament string having the intrinsic surface tenacity and tensile strength of the synthetic material, twist grooves which provide frictional resistance against slipping and an improved flexibility provided by defined spiral bores defind between the component filaments. The string of the present invention is widely used for the racket strings, fishing net strings, fishing lines and other purposes.
Another object of the present invention is to provide a method and apparatus for manufacturing the string of the invention.